1. Field of the Invention
The present invention generally relates to a cam phase detecting apparatus for an engine and a cam phase detecting method of the engine and, in particular, relates to a technique for detecting a rotating phase of a camshaft relative to a crankshaft of an engine.
2. Description of the Related Art
Japanese Laid-open (Kokai) Patent Application Publication No. 2005-291141 discloses a six-cylinder engine provided with a variable valve timing mechanism operative for varying a rotating phase of a camshaft relative to a crankshaft.
The above-mentioned six-cylinder engine is provided with a cylinder discriminating sensor and a rotating speed sensor. The former sensor has three teeth formed at even intervals on an outer periphery of a rotor mounted on the camshaft and generates a cylinder discriminating signal by the detection of each of the three teeth by means of an electromagnetic pickup. The latter sensor has a set of teeth formed angularly at each 10 deg on an outer periphery of a rotor mounted on the crankshaft and generates a unit angle signal by the detection of each of the set of teeth by means of an electromagnetic pickup.
Then, the rotating phase of the camshaft relative to the crankshaft is detected from detection of a difference in the rotating phase from an instant of generation of the cylinder discriminating signal to an instant of generation of the unit angle signal corresponding to the reference position of a piston.
Further, discrimination of a cylinder of which the associated piston arrives at the reference position thereof is executed based on whether or not generation of the cylinder discriminating signal occurs during a detection period established based on the above-mentioned unit angle signals.
As described hereinbefore, in the case where the detection of the rotating phase of the camshaft is executed based on a time difference between the detection time of a reference angle position of the camshaft and that of a reference angle position of the crankshaft, the rotating phase is detected at every instant of occurrence of the reference angle position of the camshaft.
Accordingly, there exists a problem such that, when an engine rotating speed decreases, a detection cycle of the rotating phase necessarily becomes long and therefore, feedback control of the variable valve timing mechanism cannot often be achieved at a high speed with high accuracy.